The present invention generally relates to a bar code scanner having a hologram, and particularly to a bar code scanner having a hologram which is capable of exactly reading a bar code.
Many types of laser bar code scanners have been proposed.
First, Japanese Laid-Open Patent Application No. 60-254112 discloses a laser bar code scanner. This laser bar code scanner has two hologram discs. One of the hologram discs is stacked on the other one. Each of these hologram discs has a diffraction grating formed by use of a hologram. These two hologram discs respectively have diffraction functions in which diffraction directions are opposite to each other. At least one of these hologram discs has a diffraction pattern which is symmetrical to an axis of the hologram disc. In this type of laser bar code scanner, when a wave length of a light beam emitted from a light source, such as a laser diode, changes, a position where the light beam focuses on a scanning surface is prevented from changing.
Second, Japanese Laid-Open Patent Application No. 63-218914 also discloses a laser bar code scanner. This laser bar code scanner has at least two band-shaped holograms. These band-shaped holograms are arranged so as to cross each other on a reading window. The light beam emitted from the light source passes through the band-shaped holograms so that the light beam passing through the band-shaped hologram scans a medium on which the bar code is recorded. In this laser bar code scanner, it is possible to make the structure thereof small.
Third, Japanese Laid-Open Patent Application No. 63-156293 discloses another laser bar code scanner. This laser bar code scanner forms a scanning pattern which has a plurality of scanning lines. In the plurality of scanning lines, the scanning directions differ from each other. This laser bar code scanner has a mirror group having a plurality of mirrors, each of the mirrors corresponding to one of the scanning lines, and a common mirror. Light beams reflected by the mirrors in the mirror group are respectively reflected by the common mirror so that the light beams reflected by the common mirror are directly projected onto the medium recording the bar code. In this laser bar code scanner also, it is possible to make the structure thereof small.
Fourth, a laser bar code scanner as follows has been proposed. In this laser bar code scanner, a light beam emitted from the light source is incident to a hologram formed on a rotated disc so that the the light beam passing through the hologram scans the medium recording the bar code. Then the light beam reflected by the medium is incident to the same hologram or another hologram which has the same focus position as the hologram through which the light beam from the light source passes. Finally, information is read out due to the detecting of the light beam passing through the same hologram or the other hologram above mentioned.
In the first conventional bar code scanner described above, it is possible to prevent the position where the light beam focuses on the scanning surface from changing in a direction (sub scanning direction) perpendicular to the scanning direction (main scanning direction) due to the changing of the wave length of the light beam. However, the light beam incident to the hologram disc is inclined to the surface of the hologram disc by a predetermined angle so that it is difficult to prevent the position where the light beam focuses on the scanning surface from changing in the main scanning direction due to the changing of the wave length of the light beam. Thus, when the light beam passing through the hologram discs scans the medium recording the bar code, a case occurs where the light beam discontinuously scans the medium in the main scanning direction. The discontinuous scanning causes a reading bar code error.
In the second conventional bar code scanner described above, when the number of the scanning lines which cross each other becomes large, it is necessary to stack many holograms. Thus, there are disadvantages in that it is difficult to make the holograms and the amount of light absorbed in the holograms increases.
In the third conventional bar code scanner described above, it is necessary to provide a particular optical system for transmitting the light beam to the common mirror. Thus, the variety of possible designs is restricted.
In the fourth conventional bar code scanner described above, a case occurs where the light reflected by the hologram disc is, as a disturbance light, incident to the photo detector. In order to prevent the disturbance light from being incident to the photo detector, it is necessary to increase the degree of the inclination of the light beam incident to the hologram disc. However, if the degree of the inclination of the light beam incident to the hologram disc is increased, an aberration is easily generated.